As an electrochemical cell, a redox flow battery (RFB) is a type of rechargeable battery that stores electrical energy, typically in two soluble redox pairs contained in external electrolyte tanks. An ion-selective membrane (either cation exchange membrane, CEM, or anion exchange membrane, AEM) is used to physically separate, but ionically connect, the two electrolytes that dissolve the two redox pairs. The scale of external electrolyte stored can be sized in accordance with application requirements. When needed, liquid electrolytes are pumped from storage tanks to flow-through electrodes where chemical energy is converted to electrical energy (discharge) or vice versa (charge). Different from other conventional battery systems, RFBs store electrical energy in the flowing electrolytes. Therefore, the energy capacity and the power rating are fundamentally decoupled: The energy capacity is determined by concentration and volume of electrolytes, while the power rating is determined by the size and number of cells in stack. This unique feature, combined with its long cycle-life, low capital-cost, scalability, and independence from geographical/geological limitations that are faced by pumped hydro and compressed air technologies, makes RFB one of the most intrinsically attractive technologies in electrical energy storage, especially in the field of renewable (e.g., wind or solar) electricity generation where the intrinsic intermittency has to be dealt with.
Since the first concept of RFB was put forward about 40 years ago (in 1974), significant progress has been made and some RFB systems, e.g., the all vanadium RFB (AV-RFB), have already been commercialized. However, RFBs have not reached broad market penetration yet because many challenging problems remain unsolved. For example, the generally low energy and power density of RFB have been identified to be main drawbacks when compared with other battery systems, which means more electrolyte/electrode materials are needed when certain energy capacity/power rating is required, negatively impacting their cost-effectiveness. Attempts have been made to increase the solubility of active species by choosing alternative redox pairs or using different electrolytes, which can theoretically increase the energy density, but these efforts do not improve the power density. On the other hand, efforts have been made to improve electrode performance by using better electrode designs or utilizing more active catalysts, which can increase the power density, but not the energy density. The ideal and simple solution would be the increase of RFB's cell voltage, which could increase the energy density and power density simultaneously.
A prior art RFB system 100 is shown in FIG. 1. Negative electrolyte 30 flows through negative electrode (anode) 31 from negative electrolyte source 20 via pump 15. Positive electrolyte 40 flows through positive electrode (cathode) 41 from positive electrolyte source 25 via pump 16. Positive electrode 40 and negative electrode 30 are separated by a single ion selective membrane 28. The RFB 100 may be connected to a grid input/output processor 10.
The cell voltage is simply determined by the two redox pairs used, and often the cation-based redox pairs (e.g., Co3+/Co2+ redox pair with +1.953 V standard electrode potential and Ce4+/Ce3+ one with +1.743 V, all the quoted potential values here and hereinafter calculated based on standard thermodynamic conditions) have more positive electrode potentials (ideally for the positive electrode of RFB) and the anion-based ones (e.g., Al(OH)4−/Al with 2.337 V and Zn(OH)42−/Zn with −1.216 V) have more negative electrode potentials (ideally for the negative electrode). The use of a single ion-selective membrane, either a cation exchange membrane (CEM) or anion exchange membrane (AEM), in current RFB systems theoretically requires the same ionic type of redox pairs in both positive and negative sides: either all cation-based redox pairs (when AEM used) or all anion-based ones (when CEM used), fundamentally limiting their cell voltages. For example, the earliest RFB system, i.e., the iron-chromium RFB system (Fe/Cr-RFB, [(Fe3+/Fe2+)/(Cr3+/Cr2+)] with +1.18 V standard cell voltage) and the currently most popular RFB, i.e., the AV-RFB system ([(VO2+/VO2+)/(V3+/V2+)] with +1.26 V) both belong to the all-cation-based RFB systems. The polysulphide-bromine RFB system (S/Br-RFB, [(S42−/S22−)/(Br3−/Br−)] with +1.36 V) is a typical all-anion-based RFB. Besides, the single ion-selective membrane also requires the same or similar (e.g., having the same cation but different anions when an AEM used, or having the same anion but different cations when a CEM used) supporting (or background) electrolyte in positive side and negative one, which sometimes limits the choices of redox pairs and further narrows the available range of cell voltages. For example, although the zinc-cerium RFB system (Zn/Ce-RFB, [(Zn2+/Zn)/(Ce4+/Ce3+)]) can offer as high as 2.50 V standard cell voltage (the highest number reported among all known aqueous RFB systems), it suffers a great hydrogen evolution problem in negative side (Zn2+/Zn). The reason is that the acidic supporting electrolyte used (in both sides) creates a huge over-potential (760 mV) for hydrogen evolution reaction (0 V standard electrode potential of H+/H2 at pH 0 vs. and −0.760 V standard electrode potential of Zn2+/Zn).
In addition, the use of single ion-selective membrane makes the RFB systems suffer from an irreversible counter-ion crossover that is another challenging problem, because all ion-selective membranes are not perfect. They allow a very low, but measurable, rate of permeation of counter-ions through them (typically, 1% anion crossover for CEMs and 1%-5% cation crossover for AEMs). When the counter-ions cross over the membrane, they will immediately react with the redox pairs in the other side of electrolyte (so-called self-discharging) and never come back, resulting a loss in Coulombic efficiency, permanently reduction of energy capacity, and contamination of two electrolytes which will greatly influence the performance of either side.
Thus, a suitable alternative to a single ion-selective membrane RFB system is needed.